This invention relates to tap changers for use in electrical control devices such as voltage regulators and transformers that control the transfer of voltages to loads.
A typical electric distribution system includes a power source, such as a hydroelectric dam or a coal or nuclear fired generating station, a high voltage three-phase distribution system, and electrical control devices, such as voltage regulators or transformers, to control a distribution line voltage. For example, a transformer may be used to step down the distribution line voltage to a value acceptable to the end user. The transformer includes a high voltage primary winding, a secondary winding, and a magnetic core. The high voltage winding includes a wire wound in a series of wire loops around the core, the ends of which are connected to the high voltage distribution system. The secondary winding likewise includes a series of wire loops wrapped around the metal core. The secondary winding has far fewer wire loops than the high voltage winding. Thus, the voltage induced on the secondary winding is far lower than that on the high voltage winding. The secondary winding is connected to the ultimate local load distribution system.
Although the ratio of loops in the primary and secondary coil windings does not exactly match the ratio of input or primary voltage to output or secondary voltage, the correspondence is close enough to permit fine voltage regulation on the secondary voltage side of the transformer by making slight modifications in the number of secondary loops, or windings, which are in conductive engagement with the load. This is accomplished by placing a series of leads, or taps, in conductive engagement with the secondary winding at an evenly spaced number of windings apart. For example, if a ten percent variation is required, a tap is placed on the transformer secondary winding at approximately ten percent of the windings from the end of the secondary winding. Further refinement in that ten percent variation may be accomplished by further subdividing the final ten percent of the windings with additional taps.
Variations in the load on the secondary circuit can cause corresponding variations in the voltage in the secondary circuit. For example, if the load increases, the voltage in the secondary circuit will decrease. Likewise, load decreases in the secondary circuit will increase the voltage in the secondary circuit. Such variations in line voltage can be detrimental to the performance and life of industrial equipment, and annoying to residential electricity users.
A load tap changer is used to address the load voltage variation. A load tap changer is a device that employs a secondary circuit voltage detector to actuate a mechanical linkage to selectively engage the winding taps of a tapped section of a winding, in response to voltage variations, in order to control the output voltage of a transformer or voltage regulator while under load. The tap changer may be used for controlling the voltage of a single-phase voltage regulator or of a three phase transformer.
In one general aspect, a rotary tap changer is connected to a power source to control voltage supplied from the power source to a load. The rotary tap changer includes a motor having an output device and a drive sprocket having a drive shaft positioned perpendicularly to a plane of rotation of the drive sprocket. The rotary tap changer also includes a gear engaged by the drive shaft, a first set of movable contacts, and a transmission device. The first set of movable contacts is coupled to the gear and mounted to conductively engage taps of an electrical control device. The transmission device is coupled to the motor output device and to the drive sprocket such that the motor directly drives the first set of movable contacts for selecting an electrical control device tap.
Implementations may include one or more of the following features. For example, the tap changer may include a motor sprocket attached to the motor output device. In that case, the transmission device couples to the motor output device through the motor sprocket. The drive sprocket has a first number of teeth and the motor sprocket has a second number of teeth such that a ratio of the drive sprocket number of teeth relative to motor sprocket number of teeth may be between 5:1 and 9:1. The gear may include a geneva gear. The drive sprocket and the gear may be configured such that a 360xc2x0 rotation of the drive sprocket produces a 20xc2x0 rotation of the gear. The transmission device may include a chain for engaging the drive sprocket teeth and the motor sprocket teeth.
The rotary tap changer may further include a first panel, a second panel positioned to be parallel with the first panel, and a support shaft. The support shaft is attached to the first panel and to the second panel to define an axis that is perpendicular to a plane of the first and second panels. The support shaft may support the gear. The rotary tap changer may also include a second shaft attached to the first panel to define a second axis that is perpendicular to the plane of the first and second panels such that the second shaft supports the drive sprocket.
The rotary tap changer may include a pivoting member coupled to a drive pin attached to the gear, a reversing switch configured to conductively engage a neutral tap and end taps of a tapped winding of the electrical control device through a second set of movable contacts mounted on the pivoting member to engage the reversing switch. The pivoting member may operate to select a polarity of a voltage from the electrical control device. The pivoting member may include a safety switch that trips open an electrical circuit that energizes the motor when the first set of movable contacts reaches a travel limit position. The motor may be prevented from re-energizing in a current direction of travel when the safety switch trips open the electrical circuit.
The rotary tap changer may further include a holding switch connected to the drive shaft and electrically connected to the motor. In this case, the rotary tap changer may include a control apparatus connected to the holding switch to send a signal through a first conductive path to the motor and to interrupt the signal through the first path when the holding switch closes to establish a second conductive path for selecting the electrical control device tap based on an output of the power source. The holding switch may be actuated by the drive shaft to maintain continuous power to the motor from another power source to ensure that the rotary tap changer completes a selection of the electrical control device tap after the control apparatus sends the signal to the motor. The other power source may be the power source that supplies voltage to the load. The holding switch may then be opened after a predetermined rotation of the drive shaft to de-energize the motor during selection of the electrical control device tap. The holding switch may be in series with the safety switch.
The drive sprocket may engage a device remote from the tap changer to indicate the selected electrical control device tap. The rotary tap changer may also include a second gear having an axis of rotation that is parallel to an axis of rotation of the drive sprocket and a pinion that rotates in response to rotation of the second gear to engage the device. The second gear would be engaged by an output shaft of the drive sprocket. The pinion may include a biasing device that engages the second gear to stabilize the second gear. The second gear may include a slot positioned on an outer perimeter such that the biasing device engages the slot to stabilize the second gear.
The first set of movable contacts may move from a first tap to a second tap in response to a variation in the voltage measured by a control apparatus coupled to the power source and to the load. The first set of movable contacts may move from the first tap to the second tap in a transfer time. The transfer time may correspond to one and a half cycles of a frequency of the power source. The movable contacts, motor, motor output device, drive sprocket, and the gear may be configured such that at least three current zeros occur during the transfer time. The transfer time may be less than one second or less than 500 milliseconds.
The rotary tap changer may also include a brake assembly coupled to the drive sprocket to stop the drive sprocket after the first set of movable contacts engages the electrical control device tap. The brake assembly may include a disc segment that is integral with the drive sprocket and rotates with the drive sprocket, and a stationary brake assembly. The stationary brake assembly may include brake lining strips opposing each other to define a plane that is coplanar with and centered on the disc segment. The brake lining strips may be placed under compression by a force and engage the disc segment when the disc segment passes between them. The brake lining strips may disengage the disc segment when the disc segment does not pass between them.
In another general aspect, a method of selecting a tap connected to an electrical control device for controlling voltage from a power source to a load includes receiving a signal from a control apparatus coupled to the power source to select a tap. The control apparatus couples to a motor having an output device. The motor is energized and the motor output device is rotated in response to the energization of the motor. The method includes providing a transmission device coupled to the motor output device and to a drive sprocket. The drive sprocket is driven with the transmission device in response to rotation of the motor output device. The method further includes rotating a gear in response to driving the drive sprocket, the gear engaging a first set of movable contacts. The first set of movable contacts is rotated in response to rotation of the gear to select the tap connected to the electrical control device.
The method may further include closing a holding switch connected to couple the power source to the motor to maintain continuous power to the motor from the power source to ensure that the tap is selected after the signal from the control apparatus is received. The holding switch is opened after a predetermined rotation of the drive shaft to de-energize the motor to select the electrical control device tap.
The method may also include actuating a pivoting member coupled to the gear to reverse a polarity of the tap section of a winding of the electrical control device. In that case, the method may include forming an anti-arcing bridge between a neutral tap and a second set of movable contacts connected to one end of a winding of the electrical control device. Additionally, the method may include de-energizing the motor when the first set of movable contacts reaches a travel limit position.
The method may include engaging the drive sprocket to prevent the drive sprocket from moving when the electrical control device tap is selected.
The method also includes actuating a device remote from the motor and the gear to indicate the selected electrical control device tap. Actuating the remote device may include engaging a second gear by an output shaft of the drive sprocket to rotate a pinion coupled to the remote device. The method may include stabilizing the second gear with a biasing device attached to the pinion and engaging a slot along an outer perimeter of the second gear.
The techniques and systems described here present improvements over existing load tap changers in several areas. Direct drive of the movable arcing contacts allows precise control of the transfer time from one contact to the next during a change in taps. Tap change motion is accomplished in a smooth fashion, thus avoiding impact to moving parts. A smooth transition is accomplished by storing kinetic energy in drive components to assist the motor during the change in taps. One of the advantages of the direct drive used in the tap changer is the ability to control a transfer time accurately by selecting the correct speed at which to drive the geneva gear.
A described braking system arrests the surplus kinetic energy stored in the electrical control device and stops the motion within a narrow range of angular travel of the drive components, after the tap change has been completed, without impact loads. A combination of a chain drive and geneva gear accomplishes the indexing motion of the movable contacts with simplicity and precision. In the tap changer described below, a complete tap change is accomplished in approximately 250 milliseconds for a 60 Hz AC source frequency. This fast response is beneficial during development and production where hundreds of thousands, or even millions, of testing operations are performed. A duration of a testing procedure performed on the improved tap changer is about one twentieth of the duration of a testing procedure performed with prior tap changers. For example, if prior load tap testing took about 80 days to complete, then testing for the improved load tap changer may take about 4 days to complete.
The tap changer results in longer arcing life of the contacts compared with prior tap changers in a target current range. The tap changer also provides a significant cost savings over prior tap changers. The tap changer is more reliable because of more efficient development and production. Additionally, the tap changer indexing mechanism is mechanically simpler and easier to service in the field when compared with prior tap changers.